1. Field of the Invention
The present invention relates to storage batteries, and, more particularly, to a novel lightweight electrode having improved power and energy performance for use in storage batteries and to a novel electrode and storage battery having both improved utilization and conductivity.
2. Description of the Prior Art
Despite considerable study of alternative electrochemical systems, the lead-acid battery is still the battery-of-choice for general purpose uses such as starting a vehicle, boat or airplane engine, emergency lighting, electric vehicle motive power, energy buffer storage for solar-electric energy, and field hardware whether industrial or military.
The conventional lead-acid battery is a multicell structure. Each cell contains a plurality of positive and negative plates formed of lead-based alloy grids containing layers of electrochemically active pastes. The paste on the positive plate when charged contains lead dioxide which is the positive active material and the negative plates contain a negative active material such as sponge lead. The lead-acid battery has been widely used in the automotive industry for many years and there is substantial experience and tooling in place for manufacturing this battery and its components. The lead-acid battery is based on readily available materials, is inexpensive to manufacture and is widely accepted by consumers.
There are inherent disadvantages to the lead-acid battery. During discharge of the lead-acid battery, the lead dioxide (a fairly good conductor) in the positive plate is converted to lead sulfate, an insulator. The lead sulfate can form an impervious layer encapsulating the lead dioxide particles which limits the utilization of lead dioxide to less than 50 percent of capacity, typically around 30 percent. The power output is significantly influenced by the state-of-discharge of the battery, since the lead sulfate provides a circuit resistance whenever the battery is under load. Furthermore, the lead sulfate can grow into large, hard, angular crystals, disrupting the layer of paste on the grid resulting in flaking and shedding of active material from the grid. Power consumption during charge is also increased due to the presence of the lead sulfate insulator. The lead sulfate crystals in the negative electrode can grow to a large, hard condition and, due to their insulating characteristics, are difficult to reduce to lead. Even when very thin pastes are utilized, the coating of insulating lead sulfate interferes with power output. Thus, power capability is greatly influenced by the state-of-charge of the battery.
The power and energy performance of the lead-acid battery is inherently less than optimum because most of the active material does not react in the electrochemical cycle of the battery. The active material that does not react during discharge may be viewed as dead weight which undesirably increases the weight of the battery and concomitantly decreases the energy-to-weight ratio and power-to-weight ratio of the battery. However, the active material that does not react does provide structure and conductivity for the active material that does react.
The positive plate of the lead-acid battery is the plate that normally fails in a deep cycle application. As a battery is cycled, the positive paste softens and eventually causes the battery to fail. Failure can occur in a number of ways. As the paste softens, it can lose contact with the plate and become inactive. This reduces the capacity of the battery and eventually leads to battery failure. If the softened active material falls to the bottom of the battery and bridges the gap between a positive and negative plate, the battery will fail from short circuiting.
The softening of the active material also exposes the grid to more sulfuric acid. This accelerates grid corrosion and can produce an insulating layer on the grid which prevents the active material from being in good electrical contact with the grid. In this case, the battery would fail as a result of an interface problem between the grid and active material. Grid corrosion also produces grid growth which separates the grid from the positive active material. In this case, the battery will lose capacity and eventually fail. The major problem associated with extending the life of lead-acid batteries is maintaining the integrity of the positive plate while it is cycled.
The porosity of the electrode has been recognized as essential for the discharge of the electrode so that the electrolyte is available for the chemical reaction during discharge. Attempts have been made to include in the electrode various non-conductive fillers to increase the porosity of the plates. For example, in Offenlegungsschrift DE 3631738A1 the electrode pore volume is said to be raised by the addition of non-conductive fillers in the form of micro hollow beads of glass, whose walls are made porous by etching, micro capsules of polymer or copolymers of organic materials, such as polyethylene or polypropylene which are formed as a three-quarter sphere, or hollow fibers. It is also recognized that increasing the pore volume of the electrode with these fillers reduces the conductivity of the active material. The reduction in conductivity is said to be compensated by the addition of conductive fillers, such as carbon or carbon fibers.
Japanese Patent Application No. 55-66865 discusses mixing hollow microspheres such as armosphere, philite, shirar ballons, silica ballons, and carbon ballons into the active material of the electrode to improve the discharge characteristics of the electrode.
Japanese Patent Application No. 55-108175 discusses mixing hollow microbodies as a component of the active material. The hollow microbodies are resistant to the acid in the electrolyte and form multiporous structures. The microbodies are hollow and include shells joined to cavities filled with electrolyte. The cavities are joined to the region of the plate that participates in the charging reaction.
Japanese Patent Application No. 62-160659 discusses the inclusion of hollow carbon ballons into the active material of the plate.
Another problem associated with lead-acid batteries is that the electrical conductivity for a discharged or sulphated plate is very low. Discharged portions of the plate can act to electrically isolate and prevent other portions of the plate from either charging or discharging. The utilization of the plate's active material during a discharge is reduced as a result of this electrical isolation.
Conductive fillers have been suggested for addition to the paste to improve conductivity. For example, it has been attempted to increase the conductivity of the paste by adding a conductive filler such as graphite, carbon and carbon fibers.
Graphite has been used successfully as a conductive filler in other electrochemical cells, such as in the manganese dioxide positive active paste of the common carbon-zinc cell, and mixed with the sulfur in sodium-sulfur cells. However, even though graphite is usually a fairly inert material, it is oxidized in the aggressive electrochemical environment of the lead-acid cell to acetic acid. The acetate ions combine with the lead ion to form lead acetate, a weak salt readily soluble in the sulfuric acid electrolyte. This reaction depletes the active material from the paste and ties up the lead as a salt which does not contribute to production and storage of electricity. Highly conductive metals such as copper or silver are not capable of withstanding the high potential and strong acid environment present at the positive plate of a lead-acid battery. A few electrochemically-inert metals such as platinum are reasonably stable. But the scarcity and high cost of such metals prevents their use in high volume commercial applications such as the lead-acid battery. Platinum would be a poor choice even if it could be afforded, because of its low gassing over potentials.
Hughel (U.S. Pat. No. 3,466,197) discloses the addition of 5-25 percent by weight of lead fibers to the positive paste of a deep-cycle lead-acid battery. Hughel also added 0.1 to 1 percent by weight of non-conductive polymeric fibers to increase the strength of the plates. The presence of non-conductive fibers increases bulk and weight and reduces efficiency of the plates. Furthermore, lead fibers are subject to significant stress corrosion during charge-recharge cycling. Pure lead fibers contain microcracks. Stress corrosion starts at the microcrack and continues until the fiber is consumed and loses its reinforcement function. It is very difficult to manufacture pure lead fibers without microcracking. Hughel suggests strengthening a conductive latticework by use of a tissue of lead coated glass fibers. However, no lead coated wire existed with the requisite reinforcement coating and adequate strength.
Rowlette (U.S. Pat. No. 4,507,372) discloses adding SnO.sub.2 coated glass fibers to a positive paste to maintain conductivity during charge and discharge. Again, there is an increase in bulk and loss of capacity since lead oxide is displaced with the tin oxide coated glass.
Despite the considerable effort of those skilled in the battery art to address the inherent problems of electrolyte diffusion throughout the plates and electronic conductivity of the active material in electrochemical cells, and, in particular, in a lead-acid storage battery, the art has failed to provide a solution to the long-standing problem of providing a lead-acid storage battery which satisfactorily addresses both problems at the same time in a cell to provide a lead-acid battery having improved energy-to-weight performance and high utilization at high rates of discharge. There also has been a long-standing and unsolved need for lightweight lead-acid batteries having sufficient utilization for use in the power sources of electric vehicles.
Thus, there still remains a need for an improved strong, non-corrosive electrode that is lighter in weight than a conventional electrode and that exhibits improved conductivity and utilization, and to batteries made therefrom which are especially suited for electric vehicles.
Accordingly, it is a principal object of the present invention to provide a strong, lightweight, non-corrosive electrode. It is a related object to provide such an electrode in which the active material in the plate that does not react electrochemically but provides structure and conductivity to the plate is replaced with a lightweight material that provides structure and in which there is high utilization at high rates of discharge.
It is another object of the invention to provide an electrode with improved power and energy performance. A further and related object is to provide an electrode which provides improved utilization of the active material and yet is lighter in weight than conventional electrodes.
It is yet another object of the invention to provide an electrode and storage battery having both improved utilization of the active material of the electrode and improved electronic conductivity in the electrode.
It is a specific object of the present invention to provide an electrode for a lead-acid battery which reduces the amount of active material in the electrode without compromising strength and performance. It is yet another object of the invention to provide a lead-acid battery where utilization is at least about 35% based on the one hour discharge rate.
It is a further, more specific object of the present invention to provide a lead-acid battery suitable for use in the power source of electric vehicles.
These and other objects and advantages of the present invention will become apparent from the following detailed description.